Tidal Flat Environments Research Articles

The Palaeoenvironmental Implications of the Distribution of Intertidal Foraminifera in a Tropical Australian Estuary: a Reconnaissance Study

Modern intertidal foraminifera were investigated in a mangrove‐lined microtidal distributary channel of the Barron River Delta (Cairns, Queensland, Australia). A monospecific assemblage of Trochammina inflata characterises saltmarsh environments (Biozone 1), whilst the more regularly inundated tidal flat is dominated (≥55%) by Ammonia beccarii (Biozone 2). Within the tidal flat environment, two foraminiferal subdivisions can be recognised; a high tidal flat assemblage (Biozone 2a) characterised by ≥70% Ammonia beccarii and low diversity, and a low tidal flat assemblage (Biozone 2b) with 55–65% Ammonia beccarii and high diversity (due to the settling out of small allochthonous species transported into the estuary from shelf environments). Foraminiferal distributions are also compared with tide levels estimated in the field. It appears approximately that Biozone 1 occurs between High Water and Mean High Water (MHW), Biozone 2a between MHW and Mean Tide Level (MTL), and Biozone 2b between MTL and Low Water. From this preliminary study, it is concluded that foraminifera possess significant potential in palaeoenvironmental studies of tropical Australian estuaries as indicators of intertidal environments, and also tidal levels.

Selective Feeding of the Oyster Crassostrea gigas Fed on a Natural Microphytobenthos Assemblage

In estuarine ecosystems, microphytobenthos resuspended by tidal currents often represents a large part of the food supply available to bivalves which feed selectively. This study investigated the feeding behavior of the Pacific oysterCrassostrea gigas (Thunberg) relative to a natural microphytobenthic assemblage and the effect on the structure of this assemblage. Oysters were fed only benthic microalgae collected on the intertidal mud flats of Bourgneuf Bay (France) at a suspended particulate matter concentration above the threshold of pseudofeces production. All species in the assemblage were endemic diatoms characteritic of tidal mud flat environments. Four dominant ones, which were all solitary cells with spear-shaped frustules [Navicula ammophila (Grunow),Navicula rostellata (Kutzing),Plagiotropis lepidoptera (Kuntze), andStaurophora amphioxys (Mann)] represented more than 95% of the 16 species involved. Analysis of feeding processes showed that the retention of the four main diatoms was not significantly different, but that two species, the smallest (N. ammophila [22×4 μm]) and the largest (P. lepidoptera [60×15 μm]), were preferentially ingested. The study of post-ingestive selection revealed that these two species were also preferentially digeted, i.e., preferentially directed into the digestive diverticulum, when they passed through the gut ofC. gigas. Cell size and shape did not appear to account for pre-ingestive and post-ingestive selection. The composition of the assemblage was significantly modified by oyster filtration, although the retention rates of the four main species were not significantly different. The composition of microalgae in pseudofeces and feces as a result of pre-ingestive and post-ingestive selection differed from that in seawater.

渚の危機 瀬戸内海・九州・沖縄の状況 沖縄における干潟の現状

Tidal flats in Okinawa are now facing various environmental crises. Large-scale reclamation in the southern central region of Okinawa, where many tidal flats used to exist, has continued since the 1970s. If reclamation keeps up its present pace, almost all of the tidal flats of the southern central Okinawa will disappear in the near future. Tidal flats in northern Okinawa have been greatly affected by clayey silt inflows caused by development on land. One consequence has been simplification of the tidal flat environment and a loss of benthic species diversity. Also, at Manko tidal flat, designated as a wetland of international importance under the Ramsar Convention on Wetlands, rapid sedimentation of clayey silt has led to decreases in benthic animals and plants as well as water birds. Most of the shoreline of Okinawa has been converted or is in the process of being converted to artificial breakwaters, and many forms of life restricted to sandy beach, salt-marsh and coastal wood environments exist on the extreme margin of survival. We have only scarce knowledge of the present state of most organisms living in tidal flats of Okinawa. At the same time, the status of tidal flat environments on Okinawa is worse than it has ever been.

Biostratigraphy and environment of the Holocene marine transgression in the Heligoland Channel, North Sea

A 51 m deep borehole in the outer Heligoland Channel, south-eastern North Sea, penetrated 17 m of marine Holocene, laminated, clayey silt and silty fine sand followed by 34 m of Pleistocene, fluvial sand. Samples from the two units were investigated for their content of foraminifers. The fluvial sand yielded only reworked foraminifers and, based on these is divided into two parts that can be related to differences in lithology. Based on the study of the foraminiferal assemblages in 20 samples, the marine unit is divided in five foraminiferal assemblage zones reflecting the marine inundation into this part of the North Sea after the last glaciation. The basal part of the unit is strongly influenced by freshwater and the lowermost part is dated to c. 11.0 cal. ka BP. The freshwater influence diminishes uphole with deposition of beds characteristic of a tidal flat environment in an estuarine setting with slowly increasing sedimentation depth. A subsequent sudden change in the fauna indicating higher salinity and a more open-marine environment is interpreted to reflect a change in the hydrographic situation. The basal part of this zone is dated to c. 8.6 cal. ka BP. A continuation of this trend is observed in the following zones with further increasing sedimentation depth. The fauna in the topmost assemblage zone indicates an environment comparable to the present conditions in this area of the North Sea.

Paleolimulus, an early limuline (Xiphosurida), from Pennsylvanian‐Permian Lagerst tten of Kansas and taphonomic comparison with modernLimulus

The Pennsylvanian-Permian horseshoe crab Paleolimulus signatus (Beecher), incorporating as a junior synonym P. avitus Dunbar, is one of the earliest species of the Limulina (Xiphosurida). Some specimens from Kansas, USA, are exceptionally well preserved, retaining intact book gills and appendages. Indistinct, bilobed burrowing traces of variable width occur in association with some examples of P. signatus and may have been produced by that animal. Based on actualistic taphonomic experiments on Limulus polyphemus, ancient horseshoe crabs and other arthropods having non-mineralized exoskeletons are inferred to have become pliable soon after death or moulting, and to have disarticulated slowly prior to burial. Extreme compression, wrinkling, and loose folding of sclerites are attributed to burial of a pliable exoskeleton. Slow preburial disarticulation partly accounts for the exceptional preservation of Paleolimulus remains. Also relevant for the exceptional preservation of these arthropods was burial in estuarine, tidal flat, or lacustrine environments. Because of fluctuating salinity and possibly dessicating conditions, these settings were limiting to scavengers, burrowers, and some microbes that could potentially disarticulate or decompose xiphosurid remains.

Dinosaurs nesting on tidal flats

The discovery of a dinosaur clutch in Upper Cretaceous tidal flat deposits in the Tremp Formation in Biscarri, Isona (Lleida, Spain), provides insight into the mode of life of some dinosaurs. The clutch occurs in grey marls that were originally waterlogged, muddy sediments. The seven elongated eggs are parallel to each other and with their long axis almost vertically oriented. A mound-nest is inferred in contrast to hole-nest described so far for similar eggshell types. The permeability of the eggshell (oospecies Megaloolithus siruguei) was highly enhanced by a braided network of multicanaliculate pore canals. The nest style, eggshell porosity and tidal flat environment point to a periaquatic habitat for these dinosaurs. A modern analogue occurs in crocodiles, alligators and some birds living in periaquatic environments, with nesting on plant-mounds above waterlogged soils or even on floating vegetation.

Abstract: Reservoir characterization in the San Andres formation of Vacuum field, Lea County, New Mexico - another use of the San Andres Algerita outcrop model for reservoir description

Since its discovery in 1929, Vacuum field, Lea County, New Mexico has produced 361 million bbl of oil and 298 bcf of gas from the Leonardian–Guadalupian (Permian) San Andres Formation. Despite this impressive performance, 65–70% of the estimated original oil in place remains in the ground. Complex variations in the porosity and permeability of reservoir lithologies are the principal causes for incomplete hydrocarbon recovery. Dolomitized oolitic-peloidal and fusulinid-peloidal packstones and grainstones constitute the productive reservoir facies. Interbedded in these porous units are tight, anhydritic or quartzose dolomudstones to mud-rich dolopackstones and dolomitic sandstones. Cores recovered from the nonporous intervals reveal textures, fabrics, and grain types indicative of (1) deposition in tidal-flat (peritidal) environments or (2) diagenetic modification, including collapse breccias, sinkholes, caves, and vertical fractures plugged with quartz sand or anhydrite cement, suggestive of exposure (karst) overprinting. Early geologic models of the Vacuum field were primarily generated from petrophysical data, resulting in lithostratigraphic correlations that crossed time lines and flow units. Application of a chronostratigraphic framework for the San Andres based on outcrop studies results in an updated reservoir model for Vacuum field. Improvements include (1) recognition of localized, tight, tidal-flat cycles in separate horizons in the youngest San Andres high-frequency sequence (HFS) instead of correlating all nonporous, high gamma-ray intervals as a single, regional quartzose sandstone (Lovington sand), (2) identification of bypassed pay in porous, strike-parallel dolopackstones that are stratigraphically equivalent to, but downdip from, the tidal flats, and (3) recognition of the presence of tight karst intervals in older HFSs that compartmentalize the most continuous San Andres pay interval. Deeper, uncored San Andres lithologies can be more accurately characterized by comparing petrophysical information with facies attributes from older San Andres outcrops. An untested reservoir may occur in basal San Andres skeletal dolopackstones that are sealed by dolomudstones.

Cyclic tidal successions of the Middle Ordovician Maggol Formation in the Taebaeg area, Kangwondo, Korea

Sedimentologic investigation of the Ordovician Maggol Formation in the Hyeoldong and Gumunso sections, Taebaeg area, has been carried out to delineate the depositional environment, lithostratigraphy, and cyclicity as well as to infer the paleoclimatic conditions under which the formation was deposited. Based on lithology, sedimentary structure, and fossil contents, the Maggol Formation can be divided into nine lithofacies and their corresponding depositional environments. These are 1) bioclastic/intraclastic/peloidal/oolitic packstone to grainstone (lower intertidal to subtidal); 2) biotubated limestone (subtidal); 3) lime mudstone with bioturbation and microbial lamination (lower intertidal); 4) ribbon rock (intertidal); 5) lime mudstone with microbial lamination (upper intertidal to supratidal); 6) stromatolitic limestone (shallow subtidal to lower intertidal); 7) calcareous shale (upper intertidal to supratidal); 8) fine laminated dolomite (upper intertidal to supratidal); and 9) flat pebble conglomerate (subtidal to supratidal). The lithofacies suggests that the Maggol Formation was deposited in a tidal flat environment, which shows repetitive cyclical, tidal successions. Based on the successive nature of nine lithofacies, 28 facies associations can be observed. All of the tidal successions in this study show shallowing-upward sequences, and at least 20 successions from the Gumunso area and 71 successions from the Hyeoldong area can be recognized in the outcrop. Stratigraphically, the Maggol Formation can be divided into three members based on lithology and lithofacies in this study, and this can be applied at least in the southern limb of the Baegunsan Syncline. The lower member is characterized by calcareous shale, bioturbated and/or laminated limestone, ribbon rock, and flat pebble conglomerate. The middle member is composed of about 30 m-thick, grey massive dolomite. The upper member contains bioclastic/peloidal packstone to grainstone, biotubated and/or laminated limestone, fine laminated dolomite, and solution-collapse breccia. The presence of evaporite mineral casts as well as related sedimentary structures suggests that the upper member might have been deposited in a sabkha-type tidal flat under arid climate whereas the lower member in a Bahamian-type tidal flat under humid climate.

Architecture of the tectonically influenced Sobrarbe deltaic complex in the Ainsa Basin, northern Spain

The syn-tectonic Sobrarbe deltaic complex (Eocene of the Ainsa Basin, Spanish Pyrenees) is confined by lateral thrust ramps and influenced by intra-basinal growth anticlines. Six facies associations have been distinguished within the deltaic complex: (i) slope marls and turbidite sandstones; (ii) distal delta front silty and bioturbated sandstones; (iii) proximal delta front and delta plain deposits (tidal inlet, tidal flat, mouth-bar, bay, distributary channel, and floodplain environments); (iv) biogenic deposits at flooding surfaces; (v) collapse zone sediments; and (vi) Nummulite-dominated shallow-marine carbonates. The deltaic complex comprises four composite sequences, which each contain a number of minor sequences of the genetic (maximum flooding surface bounded) type. Regressive unconformities and their correlative surfaces separate the composite sequences (CS), and each CS can be divided into lowstand, transgressive and highstand components. At the base, the Comaron CS is a WNW-prograding deltaic complex containing six minor sequences characterized by basinward-stepping sandstone wedges separated by transgressive mudstones. Double regressive unconformities are often associated with these sandstone wedges proximally in the basin. Growth of the intra-basinal Arcusa anticline caused gentle folding of the Comaron CS, and syn-growth strata at the base of the overlying Las Gorgas CS contain a prominent forced regressive sandstone wedge (the Las Gorgas sandbody) and display thinning of bedsets onto the crest of the anticline. Moreover, it can be demonstrated that relative sea-level rise took place in the growth synclines at the same time as a fall in relative sea level occurred on the growth anticlines. The tectonic deformation caused a shift in the direction of deltaic progradation towards the north-northwest, as demonstrated by the well-developed clinoforms in the highstand sequence set of the Las Gorgas CS. A second phase of tectonic activity, this time triggering large-scale collapse of the delta front, occurred at the transition between the Las Gorgas and Barranco el Solano CS. Tilting of the basin floor led to the development of coeval transgressive and regressive strata separated by a fulcrum (line of zero net-vertical movement of the shoreline). In the post-tilt phase, reduced sediment supply and accelerated subsidence led to widespread transgression and a change from clastic- to carbonate-dominated sedimentation. This phase was terminated by a gradual increase in sediment supply causing progradation of a mixed clastic–carbonate system dominated by large Nummulite banks and bioturbated shoreface marls and sandstones. Finally, a pronounced fall in relative sea level associated with the transition to the Buil CS triggered slope instabilities and eventually established fluvial conditions all over the study area. With the exception of the anticline crests, the relatively high rate of tectonic subsidence in the study area suppressed relative sea-level falls, resulting in development of discontinuous lowstand strata and bounding surfaces. The segmented nature of many regressive unconformities may be related to incremental growth of thrust-related structures, and each segment is separated from neighbouring segments by gaps of limited vertical and lateral displacement. Precursors of progressive unconformities are seen in the upper proximal part of the Comaron CS by downdip `fanning' and updip convergence of minor sequences. These differentially tilted stratal units provide reliable indicators of syn-sedimentary tectonism.

Carboniferous Mixed Terrigenous Clastic, Carbonate and Sulphate Sediments in the Bachu Area, Xinjiang

AbstractThe Carboniferous system in the Xiaohaizi area, Bachu County, Xinjiang Uygur Autonomous Region, composed of typical mixed terrigenous clastic, carbonate and sulphate sediments, was mainly deposited in the tidal flat and lagoon environments. The mixed sediments occur as the following eleven types: 1. limestone intercalated with siltstone; 2. interbeds of shale and limestone; 3. gypsolyte interbedded with limestone; 4. gypsolyte intercalated with siltstone; 5. gypsolyte interbedded with shale; 6. gypsolyte intercalated with siltstone, limestone and dolomite; 7. siltstone interbedded with gypsolyte and limestone; 8. terrigenous detritus scattered in carbonate matrix; 9. carbonate as cement in clastic rocks; 10. mixed sediments of carbonate and terrigenous mud; 11. mixed sediments of carbonate grains with terrigenous sand. Based on the analysis of the dynamic mechanism of mixed sediments, it is believed that these types of mixed sediments in the study area were controlled by climate, sea level change and sources of sediments.

The Role of Sand /Shale Interfaces in Saturated Zone NAPL Trapping

Abstract Spilled nonaqueous phase liquids (NAPLs) can be trapped by relatively small-scale structures at sand/shale interfaces just as hydrocarbons are naturally trapped in larger scale structures in oil and gas fields. We measured microscale closure at the upper sand/shale interface for 62 locations at three sites for fluvial/alluvial and tidal flat depositional environments and found that closure volume ranged from 0.56 to 2.9 L/m2 and averaged 1.2 L/m2. The area of each measurement ranged between 1024 and 2090 cm2. We could not measure closure height for individual features, so we measured the topographic variation at each of the 62 locations. Topographic variation ranged up to a maximum of 3.3 cm, and site means ranged between 1.3 and 2.4 cm. We also conducted sand box experiments to see if microscale features have NAPL trapping potential. Our results suggest that for NAPL to be trapped at greater than residual saturation, the trap closure height must be greater than the capillary rise of water into the NAPL. A closure height of 4.0 cm effectively trapped Marvel Mystery Oil when the average grain size was 1.5 mm but failed to do so at smaller grain sizes. Our experimental results are reproduced adequately by Hobson’s Formula which states that the height of capillary rise is inversely proportional to grain size and is also a function of the physical properties of the NAPL. Further calculations using Hobson’s Formula suggest that traps containing coarse-grained material are effective for some NAPLs at closure heights approaching 1 cm, whereas fine-grained sands require minimum closure heights of slightly <1 to 5 m or more to trap NAPLs.

Origin and modification of early dolomites in cyclic shallow platform carbonates, Yeongheung Formation (middle Ordovician), Korea

The middle Ordovician Yeongheung Formation, Korea, consists of numerous metre-scale, shallowing-upward cycles, which were deposited on a shallow-marine carbonate platform. Depositional sequences developed on the inner platform were completely dolomitized regardless of constituting lithology, whereas those developed on the outer platform were partially dolomitized. The amount of replacement dolomite within a metre-scale, shallowing-upward peritidal cycle decreases downward from the peritidal cycle cap in the partially dolomitized depositional sequences. The cross-platform distribution of dolomites and their stratigraphic position in the sequences suggest that the Yeongheung dolomite formed syndepositionally. The common existence of pseudomorphs of evaporite minerals and nodules in the Yeongheung Formation indicates that dolomitization of the Yeongheung Formation occurred in a sabkha-type tidal-flat environment. It also occurred during each cycle of deposition influenced by high-frequency sea-level oscillations. The present-day Yeongheung replacement dolomites are characterized by near-stoichiometric to stoichiometric composition (49.9 to 52.1 CaCO 3 mole%), and low strontium concentration (52 to 133 ppm). The δ 18O value of the replacement dolomites ranges from −7.7 to −5.2‰ (PDB); this is much depleted relative to that of contemporary sabkha-type dolomites. These geochemical characteristics of Yeongheung dolomite indicate significant post-depositional modification. The common existence of oomolds, leached skeletal grains and dissolved evaporites in the Yeongheung Formation illustrates invasion of meteoric water into the sediments. Recrystallization of Yeongheung dolomites in the infiltrated meteoric water at slightly elevated temperatures during shallow burial resulted in lighter δ 18O values and lower strontium concentration.

The impact of wildfire on an Early Carboniferous coastal environment, North Mayo, Ireland

At Creevagh Head, near Ballycastle, North Mayo, Ireland, a 28-m sequence of Lower Carboniferous (Holkerian-Arundian) carbonates and clastics is exposed in low sea-cliffs. These rocks have been described in terms of five facies and were deposited in a coastal marine setting. Horizontally laminated heterolithic siltstones were formed in a tidal flat environment (Facies 1). Lithologically similar deposits infilling a broad (150 m wide), shallow (2 m deep) erosional surface represent the sediments of a major estuarine channel (Facies 2). Units of inclined heterolithic stratification (IHS), 2–4 m thick, displaying arcuate structures in plan, record the migration of meandering estuarine creeks which dissected the tidal flat (Facies 3). Beds of carbonate mudstone bearing abundant Volvocacean green algae and organic rich siltstones probably formed in oligohaline coastal lagoons (Facies 4). Finally, bioclastic sandstones with in situ corals formed in stenohaline waters at the mouth of tidally dominated estuaries (Facies 5). Vast quantities of charcoal present in one of the IHS units (the Charcoal Unit) are interpreted as the product of a single, catastrophic wildfire at this time. The fire appears to have destroyed vegetation cover on the hinterland destabilising local sedimentary, hydrological and biochemical systems. Comparison of the Charcoal Unit with IHS units immediately underlying it, suggest that run-off and sediment discharge were increased greatly downstream following the fire, replacing tide-dominated sedimentation with a river-dominated regime. Abundant fish skeletal remains in the Charcoal Unit indicate that this event coincided with massive mortalities amongst marine fish populations. This ecological catastrophe may have been linked to increased turbidity and nutrient flushing into the estuary, causing eutrophication and short-lived anoxia.

A sandy tidal coast in the uppermost Muschelkalk and the origin of the Muschelkalk/Keuper boundary in the southwestern Germanic basin

A tidal coast is documented in mixed siliciclastic and carbonate sands of the uppermost Muschelkalk (Middle Triassic) along the southwestern margin of the Germanic basin in Luxembourg. The coastal sediments are vertically and laterally stacked channel fills, interpreted to have formed in a tidal flat environment. The channel fills overlie carbonates of a shallow subtidal ramp. The strong progradation of the tidal flat indicates deposition during a late stage of sea-level highstand, but before sea-level fall. In their upper part, the channel fills are overprinted by a thick paleosol, which resulted from subaerial exposure around the time of the Muschelkalk/Keuper boundary. The exposure and formation of the paleosol in the subtidal coastal sediments and, in basinward sections, the deposition of dolomicrites above the Muschelkalk/ Keuper boundary in the lowermost Keuper both indicate a sea-level fall.

Facies analysis of an Upper Cretaceous high-energy rudist-dominated carbonate ramp (Matese Mountains, central-southern Italy): subtidal and peritidal cycles

Upper Cretaceous shallow-water limestones rich in rudists have been studied in the Matese Mountains. They show a preponderance of skeletal components (molluscs and benthic foraminifers) and a lack of non-skeletal grains. The most prominent sediments of the studied stratigraphic interval are rudist-dominated fine- to coarse-grained lithologies. Most of the Senonian successions are characterized by massive neritic limestones rich in molluscs (mostly rudists and subordinate gastropods) and a few corals. The sediments were generated in situ on shelves where rudist bivalves were the primary sediment producers. These sediments were actively moved by storms, waves and swells. The finer fractions were probably winnowed out and deposited in deeper water. Well-bedded dolomitized mudstones/wackestones, microbial laminites and benthic foraminifer wackestones characterize some successions where they rhythmically alternate and testify to deposition in tidal-flat environments. The lithofacies, faunal and taphonomic characters and the sedimentary structures are typical of a ramp-like open shelf with local peritidal deposition. The vertical lithofacies organization consists of numerous decimetre- to metre-scale shallowing-upward subtidal and peritidal cycles. A complete transition from peritidal to subtidal cycles has been recognized and documented. The peritidal cycles are usually bounded by desiccation surfaces and/or dolomitized crusts followed by shallow-subtidal to peritidal facies of the next cycle. The subtidal cycles are typified by rudist floatstones passing to large-scale cross-bedded bioclastic packstones/grainstones. Current and storm structures are distinctive features; in shallower-water environments the effects of submarine erosion prevail; hardgrounds commonly developed and the resulting cycles are usually truncated. The formation of the peritidal cycles appears to have been controlled by accommodation space in response to fluctuations in relative sea level. The formation of the subtidal cycles was determined by sedimentation rates in response to fluctuations in the zones of fair-weather and storm-wave reworking, in turn controlled by relative sea level. This indicates that sediment production and supply were important controlling factors in the cycle formation during the early Senonian stratigraphic interval. The recognition of similar depositional styles in other early Senonian successions cropping out in the southern Apennines suggests that this type of sedimentation is a recurring attribute of the Upper Cretaceous open shelves.

The Backbarrier Sediments of the Skallingen Peninsula, Denmark

Geografisk Tidsskrift, Danish Journal of Geography 97: 11–32, 1997. The Skallingen peninsula is formed by sand derived from the south-going littoral drift along the North Sea coast. The backbarrier has accumulated sand as a result of wash-over deposits. In the beginning of the 20th century, fine grained sediment started to cover the surface in form of salt marsh deposits. The sand on the peninsula consists primarily of fine sand with a mean grain size of 2.5–2.25 phi (0.177–0.210 mm). Exposed to waves in the tidal flat environment and to tidal currents in the salt marsh creek environment, the finest sand fractions are removed and deposited on the salt marsh. The remaining sand is sorted in accordance with the dynamics in the sub-environments. The change in grain-size characteristics during this dispersal of sand in the back barrier is analyzed by means of decomposition of the grain-size distributions into overlapping log-normal distributions. The import of fine grained sediment is demonstrated, based on measurements of tidal dynamics and concentration of suspended matter in the mouth of a salt marsh creek. The salt marsh formation is analyzed on the basis of210Pb-datings and grain-size analysis of the surface layers in a line across the central part of the backbarrier environment. The deposition rate is observed to increase from south to north, when approaching the inner part of the tidal area. The grain-size distribution of the average material depositing on the backbarrier in the central part of the peninsula consists of 15% sand, 46% silt and 39% clay with a corresponding deposition rate of 0.07 g.cm−2 y−1. The dry density of the salt marsh clay is primarily controlled by variations in the content of organic matter and by compaction. Two power functions are formulated to describe variations in the dry density: D = 4.47·O−0.81; 10<O<50; r=0.98 (surface samples) and D = 0.58*X 0.17; 0.5<X<32.5; r=0.70, where D is the dry density (g/cm3), O is loss in ignition (%) and X depth below the surface.

Trace-element and UPb isotope compositions of pyrite types in the Proterozoic Black Reef, Transvaal Sequence, South Africa: Implications on genesis and age

The Proterozoic Black Reef is, in most cases, indistinguishable from the gold-, uranium- and pyrite-bearing Archaean Witwatersrand reefs. However, the prevailing opinion that the Black Reef gold mineralization was largely derived from reworking of the underlying Kimberley Reef is not supported by the morphology, trace-element geochemistry and UPb isotope data on detrital pyrite in these reefs. The grade of metamorphism recorded in the Black Reef Quartzite Formation is significantly lower than the 300–400°C that has been proposed for the sediments of the Witwatersrand Supergroup. The UPb isotope and trace-element geochemical signatures in pyrite have, therefore, been preserved in the Black Reef. The morphological and compositional differences retained by the detrital and synsedimentary pyrite types, both occurring together in the Black Reef, imply that they were formed by different processes and in contrasting geochemical environments. In the Black Reef, the data for the detrital pyrite are consistent with their derivation from a ∼ 3000-2800-Ma granitic, probably hydrothermally altered, source region. By contrast, the geochemical and Pb-isotopic composition of the concretionary pyrite is interpreted to reflect its synsedimentary formation in a lagoonal or tidal flat environment prior to ∼ 2500 Ma. The morphology, primary mineral inclusions and the unradiogenic Pb-isotopic compositions preserved in the Black Reef detrital pyrite are irreconcilable with models proposed for the formation of Witwatersrand pyrite by a process of pyritization of Fe-bearing detrital minerals. Fluids present in the Black Reef depositional/diagenetic environment were anomalously enriched in radiogenic Pb and any transformation process would not be able to escape acquiring this enriched Pb-isotopic signature.

Sequence stratigraphy and stable isotopes ( δ13C, δ180) of the Late Cretaceous carbonate ramp of the western margin of the Iberian Chain (Soria, Spain)

Within the Upper Cretaceous sediments of the western margin of the northern Iberian Chain, several formations of Turonian to Campanian age form a carbonate platform succession (165–270 m thick) developed during a second-order cycle of relative sea-level rise and fall. This succession consists of two depositional sequences, deposited during third-order cycles, formed by sediments deposited in a broad, epeiric, ramp-type platform open to the north. The lower depositional sequence (Moral de Homuez Formation) began during the Cenomanian-Turonian transgression and represents the transgressive systems tract and highstand systems tract; it was terminated by a late Turonian regression resulting in a type 2 sequence boundary. Two types of parasequences (deepening-upward and shallowing-upward, respectively), 1 to 3 m thick, deposited within fourth- and higher-order cycles, have been identified, with features corresponding to lagoonal deposits. The upper depositional sequence (comprising the Hortezuelos, Caballar, Hontoria del Pinar, Burgo de Osma and Santo Domingo de Silos Formations) began during the early Coniacian transgression and was terminated by a Campanian regression. This upper depositional sequence also represents the transgressive systems tract and highstand systems tract. Eight types of shallowing-upward parasequence, 1 to 4 m thick (locally up to 10 m), deposited in subtidal high-energy (shoals and channels) and low-energy settings (lagoon, locally with rudist patch-reefs) and in tidal-flat environments have been identified. Carbon-isotope data, showing sharp and correlatable positive anomalies in different sections, strongly suggest that Oceanic Anoxic Events are recorded in these shallow platform deposits. The Cenomanian-Turonian Oceanic Anoxic Event results in a positive δ 13C excursion of up to 4‰ that coincides with the presence of small-size, low-diversity benthic foraminifers, agglutinated and planktonic foraminifers and calcispheres, despite the paucity or complete absence of black shales. The Santonian-Campanian Oceanic Anoxic Event is recorded but is not so evident. The relative δ 13C minimum between these two maxima apparently coincides with the Turonian-Coniacian boundary. It is concluded that short-term variations in δ 13C can be useful for correlation in shallow platform areas, where the fossil record has little biostratigraphical precision. Accordingly, the ages previously assigned to the sediments studied have been partially modified. The oxygen-isotope results are more negative than expected for carbonates precipitated in equilibrium with normal marine waters, thus indicating diagenetic alteration by meteoric waters.

1.66-Ga-old metamorphosed Pb-Cu deposit in Sargipali (eastern India): manifestations of tidal flat environment and sedex-type genesis

The metasedimentary rocks and the unusual PbCu mineralization of the Sargipali mine area are described and discussed to elucidate the processes involved in their deposition. Important lines of evidences used for the interpretation of an inter- to supratidal flat environment are: (1) a general pattern of a shallowing upward sequence from coarse clastics (arenaceous) through finer clastics (argillaceous) to carbonate; (2) the occurrence of metaconglomerate, trough cross-bedding and wave-ripples (within the arenaceous lithostratigraphic unit); (3) intercalations of quartzite, carbonate skarn, dolomite, and convolution of chert laminae (within the argillaceous lithostratigraphic unit); and (4) intercalation of quartz-mica-chlorite schist laminae (within the carbonate lithostratigraphic unit). The ores seems to belong to the same environment. Furthermore, in the assessment of the origin of the Sargipali ores, it is recognized that the orebodies not only represent a sedimentary setting, but also show a high Pb-content, and an older age of mineralization than that of deformation, metamorphism and granite intrusion. Along with the lack of stockwork zones, wall rock alteration, subaerial volcanism, and the presence of chert and tourmalinite, these features are consistent with a sedex-type origin. The idea of this origin finds support by yet another observation, i.e., the ore sulphur was mainly derived from the inorganic reduction of the seawater sulphate (at more than 300°C), while the ore metals were leached from the μ (= 238U/ 204Pb)- enriched Singhbhum soda-granite of 2200 Ma.The present study may help in conceiving a ‘new’ possibility that the sediment-hosted Pb-rich deposits of MVT origin can also be evaluated in terms of the sedex model.

Dissocladella bystrickyi n.sp., a New Calcareous Alga (Dasycladaceae) from Upper Triassic Dolomites of Mt. Medvednica (Northern Croatia)

Dissocladella bystrickyi n.sp. occurs in early diagenetic Upper Triassic dolomites, that were deposited in tidal flat and tidal channel environments in the carbonate platform interior. The new alga, assigned to the genus Dissocladella, is characterized by verticillate and distally flattened primary ramifications which give rectangular pores and each bear 5-7 club-shaped secondaries. Its stratigraphic position is probably within the Norian-Rhaetian.